function showMenu()
{
    var subMenu = document.getElementById( 'sub_menu' );
    subMenu.style.display = "block";
}

function hideMenu()
{
    var subMenu = document.getElementById( 'sub_menu' );
    subMenu.style.display = "none";
}

function initializeDropDownMenu()
{
    var menu    = document.getElementById( 'menu' );
    var subMenu = document.getElementById( 'sub_menu' );
    
    menu.onmouseover = showMenu;
    menu.onmouseout  = hideMenu;
    
    subMenu.onmouseover = showMenu;
    subMenu.onmouseout  = hideMenu;	
}